1. Technical Field
This disclosure relates to a fixing device and an image forming apparatus including the fixing device, and more specifically to a fixing device to fix a toner image on a recording medium by heat and pressure and an image forming apparatus, such as a facsimile machine, a printer, a copier, or a multi-functional device having at least one of the foregoing capabilities, which includes the fixing device and employs, e.g., an electrophotographic or electrostatic recording method.
2. Description of the Related Art
Image forming apparatuses are used as printers, facsimile machines, copiers, plotters, or multi-functional devices having, e.g., two or more of the foregoing capabilities. As one type of image forming apparatuses, electrophotographic image forming apparatuses are known. For example, an electrophotographic image forming apparatus forms an electrostatic latent image on a surface of a photoconductor drum serving as an image bearer, develops the latent image on the photoconductor drum with, e.g., toner serving as developing agent to form a visible image, transfers the toner image onto a recording sheet (also referred to as recording medium or sheet of paper) by a transfer device, and fixes the toner image on the recording sheet by a fixing device.
Such a fixing device may have a fixing member and a pressing member, which are formed with, e.g., an opposing roller(s), a belt(s), and/or a combination thereof. The pressing member is configured to contact the fixing member to form a nipping portion between the pressing member and the fixing member. The fixing member and the pressing member sandwich a recording sheet at the nipping portion, and heat and pressure are applied to fix a toner image on the recording sheet.
For example, a technique is known using a fixing belt extending between roller members as a fixing member (e.g., see JP-H11-002982-A). As illustrated in FIG. 1, the fixing device includes, for example, a fixing belt (endless belt) 204 serving as a fixing member, multiple roller members 202 and 203, a heater 201, and a pressing roller (pressing member) 205. The roller members 202 and 203 support the fixing belt 204, and the fixing belt 204 is wound around the roller members 202 and 203. The heater 201 is disposed inside one (the roller member 202) of the roller members 202 and 203. The heater 201 heats the fixing belt 204 via the roller member 202. When a recording medium P is fed toward a fixing nip portion (nipping portion) N between the fixing belt 204 and the pressure roller 205, heat and pressure are applied to a toner image on the recording medium P to fix the toner image on the recording medium P (belt fixing method).
Some fixing devices have a stationary member to slidingly contact an inner surface of a fixing member serving as a rotary body. For example, JP-H04-044075 proposes a fixing device using a film heating method. As illustrated in FIG. 2, such a fixing device includes, e.g., a ceramic heater 211, a pressing roller 212, and a heatproof film (fixing film) 213. The ceramic heater 211 serves as a heat generator and the pressing roller 212 serves as a pressing member. The ceramic heater 211 and the pressing roller 212 sandwich the film 213 and form a fixing nip portion (nipping portion) N. A recording medium bearing a toner image to be fixed is introduced between the film 213 and the pressing roller 212 in the nipping portion N, and conveyed with the recording medium P sandwiched together the film 213. Heat of the ceramic heater 211 is applied to the recording medium at the nipping portion N via the film 213. Additionally, pressing force is applied to the recording medium at the nipping portion N to fix the toner image on the recording medium.
Such a fixing device using the film heating method can be an on-demand type of fixing device using a small heat capacity of members as the ceramic heater and the heatproof film. Only when an image forming apparatus including the fixing device performs image formation, the ceramic heater serving as heat source is electrified to generate heat to a desired fixing temperature. Such a fixing device is advantageous in a relatively short waiting time (quick start) from when the image forming apparatus is powered on to when the image forming apparatus turns into an executable state for image formation, a relatively small power consumption in the standby state (power saving), and so on.
Alternatively, for example, JP-H08-262903-A and JP-H10-213984-A propose image fixing devices using pressure belt methods. Such a fixing device may have a heat fixing roll, an endless belt, and a pressing pad. The heat fixing roll is rotatable and has an elastically deformable surface. The endless belt (pressing belt) is movable while contacting the heat fixing roll. The pressing pad is disposed in non-rotational state inside a loop of the endless belt to press the endless belt against the heat fixing roll to form a belt nip between the endless belt and the heat fixing roll. A recording sheet is passed through the belt nip. The pressing pad elastically deforms the surface of the heat fixing roll. For such a fixing method, a belt is used as a lower pressing member to increase a contact area between a recording sheet and a roll. Such a configuration enhances the efficiency of heat conduction, suppresses energy consumption, and allows downsizing.
For example, the above-described fixing device like that described JP-H11-002982-A is advantageous in speedup over a fixing device using a fixing roller. However, the fixing device are disadvantageous in reducing a warm-up time (a time required to reach a printable temperature) or a first print time (a time required to, after reception of a print request, prepare printing, perform print operation, and output a printed sheet).
By contrast, the fixing device described JP-H04-044075 has a relatively small heat capacity, thus allowing downsizing and a reduction in the warm-up time and/or the first print time. However, the fixing device described JP-H04-044075 is disadvantageous in durability and temperature stability of the belt.
In other words, the fixing device described JP-H04-044075 may be insufficient in wearing resistance to the sliding of the fixing belt over the ceramic heater. When the fixing device is driven for a long time, friction against the fixing belt is continuously repeated, thus causing a rough surface of the fixing belt. As a result, friction resistance is increased, thus resulting in unstable running of the belt or an increase in driving torque of the fixing device. Consequently, a transfer sheet on which an image is formed may slip on the fixing belt, thus causing displacement of the image. Additionally, stress to a driving gear may increase, thus giving damage to the driving gear.
In addition, for the fixing device using the film heating method, since the belt is intensively heated at the nipping portion, the temperature of the belt becomes lowest when the belt in rotation returns to an entry to the nipping portion. As a result, in particularly, when the belt is rotated at high speed, a fixing failure may occur.
By contrast, for JP-H08-262903-A, an outer surface of the pressing pad includes a polytetrafluoroethylene (PTFE) impregnated glass-fiber sheet (PTFE-impregnated glass cloth) as a low friction sheet (sheet-type sliding member) to improve the sliding performance of the inner circumferential surface of the belt and the stationary member. However, for such pressure-belt type fixing devices (described in JP-H08-262903-A and JP-H10-213984-A), the fixing roller may have a relatively large heat capacity, thus increasing the warm-up time.
In light of the above-described challenges, for example, JP-2007-334205-A proposes a fixing device including an endless fixing member, an opposed member and a resistant heat generator. The opposed member (metal heat conductor or support member) has a substantially tubular shape and is disposed at an inner circumferential side of the endless fixing member. The resistant heat generator is, e.g., a ceramic heater disposed at an inner circumferential side of the opposing member to heat the opposing member. Such a configuration allows heating of the entire fixing belt and a reduction in the warm-up time and the first print time, and prevents deficiency of the heat amount during high speed rotation.
However, for the fixing device described in JP-2007-334205-A, a pressing roller serving as a pressing member is pressed toward the fixing belt to form a nipping portion, and the nipping portion is supported by the metal heat conductor. Such a configuration may be unstable in the width and pressure of the nipping portion N.
Hence, to stably retain a state, a shape, and/or a position of the nipping portion formed by the fixing belt and the pressure roller or a tubular heating member, for example, JP-2010-096782-A proposes a configuration in which, e.g., a nip formation member (contact member or stationary member) and a reinforcement member are disposed corresponding to the position of the nipping portion.
The pressure roller is pressed against the nip formation member via the fixing belt, and the nip formation member slidably contacts the fixing belt rotated in a circumferential direction. Therefore, a surface of the nip formation member preferably has a low friction relative to the fixing belt. Additionally, to obtain a high quality image, the nip formation member preferably has a relatively high elasticity like rubber to follow minute irregularities of a surface of the recording medium. However, since rubber typically has a high friction relative to the fixing belt, it may be difficult to use rubber as a material of the surface of the nip formation member.
Hence, by mounting a sliding sheet (low friction sheet) on the surface of the nip formation member, both high elasticity like rubber and low friction can be obtained. However, because of rotation of the fixing belt, the sliding sheet continuously receives shearing stress at a downstream side in a rotation direction of the fixing belt. Therefore, the sliding sheet is preferably firmly fixed on a face of the nip formation member facing the nipping portion.
For example, as a technique of fixing the sliding sheet to the nip formation member, the sliding sheet is wound around the nip formation member and screw holes are formed in an opposite face of the nip formation member opposite the nipping portion to fasten the sliding sheet with screws. Alternatively, JP-2011-070070-A proposes a method in which a nip formation member has an engaging structure including a projecting portion and a recessed portion to sandwich and fix the sliding sheet between the projecting portion and the recessed portion. However, such configurations of the above-described technique and JP-2011-070070-A increase the number of components of the nip formation member, thus increasing the production cost.